1. Field of the Invention
This invention relates to a vehicle suspension and, more particularly, to a suspension for allowing front and rear portions of a vehicle to rotate with respect to each other about an axis which extends above the front portion's center of gravity.
2. Description of the Prior Art
The construction and logging industries often use vehicles which must be capable of traveling over rugged, uneven terrain covered with such debris as fallen trees, stumps and rocks. Many of these vehicles utilize an articulated frame whereby steering is accomplished by providing a vertical pivot axis at some point along the length of the vehicle.
In order to maintain all four wheels in contact with the ground, both articulated and nonarticulated vehicles must be provided with a suspension capable of allowing relative vertical movement between the wheels.
One system for performing this function is the independent suspension, in which vertical movement of each of the wheels is independent of the others. However, such suspensions are relatively expensive, and are generally not capable of providing sufficient clearance to allow the vehicle to travel over large obstacles.
Another type of suspension is the trunion-mounted axle wherein the entire axle pivots with respect to the chassis, about a horizontal axis such that as the wheel on one side of the vehicle moves vertically in one direction, the wheel on the opposite side of the vehicle moves the same distance in the opposite direction. A principal disadvantage of this type of suspension is that the mass of the vehicle is free to pivot from side to side, substantially reducing the stability of the vehicle. Such vehicles tend to tip easily, particularly in response to side forces as might be generated when a rear-mounted logging winch is pulling a log which is laterally offset from the vehicle.
A third type of suspension which overcomes many of the aforementioned disadvantages is the oscillating frame suspension. In this type suspension, the vehicle includes two chassis fastened to each other through an oscillating connector which allows each chassis to rotate with respect to the other along a substantially horizontal axis. In the oscillating frame suspension, both axles are immovably secured to their respective chassis, and relative vertical movement of one wheel with respect to the others is accomplished by rotation of the entire chassis. There is a serious safety problem with this system, however, that is not present in the aforementioned suspension systems. Since the axis of rotation between the two chassis passes beneath their respective centers of gravity, both chassis are inherently unstable and, should the wheels of one chassis lose contact with the ground, the raised chassis will rotate to one side about the rotational axis.